Guest who wonders why it's called the delta when it's not one:   

It's my understanding that in the 19th century, the low alluvial plain country that surrounds the Lower Mississippi had a topography which looked remarkably similar to the Nile Delta. This inspired the naming of Cairo, Illinois and Memphis, Tennessee, etc. Ignorant? I guess it seems so, to those of us with instant access to Google Earth, and after the work done by Fisk etc. in the 1940s to unravel the story of the shifting river and its floodplains and deltas, which by the way it's hard to see as you travel where the plains stop and the deltas begin.

By the time there was greater scientific understanding the place had a name which was well ingrained in common use.